Functional distribution of coronary vascular volume in beating goat hearts

With use of hemoglobin-bound O2 as an endogenous tracer, intramyocardial blood volume distribution between vessels involved in O2 exchange and more distal vessels was estimated. In nine anesthetized open-chest goats, the left main coronary artery was cannulated and perfused at a constant flow. Coronary arteriovenous O2 content difference (AVOX) was measured. AVOX transients induced by a flow step could be characterized by two phases: delay time and slow change to a new steady state. AVOX responses were fitted by a two-compartment model consisting of a well-mixed compartment from which O2 is consumed with volume Vm, and a distal unmixed compartment without O2 exchange, with volume Vunm. The rate of change of the AVOX transient depends on Vm, whereas the delay time depends on Vunm. Measurements in nine goats resulted in a Vm value of 9.9 +/- 1.1 (SE) ml/100 g (n = 9) and a Vunm value of 3.8 +/- 0.3 ml/100 g. Maximal vasodilation caused a significant increase in Vm (13.1 +/- 1.3 ml/100 g; n = 5), whereas Vunm did not change (5.0 +/- 0.5 ml/100 g). Hence, the increase of intramyocardial blood volume induced by vasodilation must be expected in the capillary bed and not in the coronary veins.